It is known in the art to shape the discharge vessel as a longitudinally extending cylinder or as a vessel that bulges out in the center for sodium high-pressure discharge lamps, whereby the inner diameter of the discharge volume is greater than that at the ends. It is particularly taught that the inner diameter at the level of the electrode tip amounts to at least 60% of the inner diameter in the center.
A discharge vessel also is known which is shaped from a straight cylindrical tube, which possesses ends with reduced diameter. The cylindrical tube can have an elliptical cross section. Alternatively, a very longitudinally extended elliptical discharge vessel also is known, whereby the axis ratio amounts to 1:4 to 1:8.
In the case of such longitudinally extended discharge vessels, a universal burning position is not possible when the filling contains metal halides. In the vertical burning position, the cold-spot temperature, which is found in the region of the lower electrode, is clearly lower than for the horizontally burning lamp. As a consequence, there is a pronounced color shift between horizontal and vertical burning positions. Further, the temperature distribution is relatively inhomogeneous in the case of such longitudinally extended geometries of the discharge vessel, so that a more intense temperature gradient occurs. In the case of a pre-selected cold-spot temperature (which is necessary for achieving the aimed-at light-technical values), a very high hot-spot temperature is established in the case of longitudinally extended geometry, which can lead to an overloading of the ceramics of the discharge vessel.
A cylindrical discharge vessel with end surfaces applied at right angles is known, in which the electrodes are inserted in a recessed position in the ends. Such cylindrical discharge vessels in fact permit a universal burning position, but their temperature distribution is also inhomogeneous, so that here also, a very high hot-spot temperature arises.
A high temperature gradient, as is formed both in longitudinally extended elliptical as well as cylindrical discharge vessels, favors corrosion phenomena on the ceramics during the service life of the lamp.
In addition, the principal possibility given by the use of ceramics, to increase the cold-spot temperature in comparison to quartz glass and thus to improve the light-technical data, is limited in these geometries by the very high hot-spot temperature that occurs therein. The hot-spot temperature of the ceramics is limited maximally to approximately 1250.degree. C., if service lives of 6,000 to 10,000 hours are aimed at.
It has also resulted from this that in the case of such longitudinally extended cylindrical or elliptical discharge vessels, the light-technical and electrical lamp data are greatly dependent on burning position, due to their very inhomogeneous temperature distribution. Such discharge vessels can thus only be applied, if it is not required that these lamp data be independent of burning position. This is only possible for lamps with a base on both sides. Normally, only a horizontal burning position is possible for them.